Chap. XX. Of the Drill-Boxes. 315 



acute Triangle, whofe vertical Angle is more or lefs 

 acute, according as the Tongue approaches nearer to, 

 or recedes farther from the Spindle. 



This Fig. 1. Plate 3. is the brafs Tongue with its 

 Backrfide upwards. The Two outer Lines a b and 

 c d are the Edges of the upper Surface (tho' turned 

 downwards in this Figure), which are inclined to each 

 other, as afore- mentioned-, but the Two inner Lines 

 e f and g h are nearer to each other, whereby this 

 under Surface is narrower than the upper: Both muft 

 be plain Surfaces, but the upper and its Two Edges 

 yery free from Flaws, and lmooth, or polifhed. 



The Reafon why the under Surface is narrower 

 than the upper, is to preferve the Bevel of the empty 

 Triangle : For though the Bevel of the Sides of the 

 Mortife would be fufficient for this, if both Sides 

 of the Tongue were fure to keep equally diftant from 

 the Spindle ; yet as the Tongue never is fo tight on 

 its Axis, but that fometimes one Corner of it may 

 be nearer to the Spindle than the other, in this Cafe, 

 that Side which is neareft to the Spindle would reverie 

 that Bevel, fo as to make the fmall empty Space that 

 is betwixt the Mortife and the Tongue, wider above 

 than underneath. 



C C are the Two little Knobs that prevent the 

 Spring from flipping to either Side, and are at the 

 Diftance from one another of the Breadth of the 

 Spring. 



Fig. 2. fnews one Side, and the Thicknefs of the 

 Tongue the other Side, being the fame, a b mews 

 the polifhed Surface (being a true Plane,), whereon 

 the Seed runs down to the Spindle, c d the Back- 

 fide, which lies turned uppermoft in Fig. 1. bed 

 fhews one End of the hollo w Cylinder of the Tongue, 

 thro 3 which its Axis paries. 



The Length of the Tongue mud be fuch, as will 

 reach lower than juft to touch the Bottom of the 



great 



